Steerable magnetic dipole antenna for measurement while drilling applications

ABSTRACT

A steerable, magnetic dipole antenna for Measurement-While-Drilling (MWD) or Logging-While-Drilling (LWD) applications. The antenna elements use a hole arrangement in addition to grooves in a steel tool body, which is typically a drill collar. This antenna embodiment is extremely robust, meaning that does not significantly reduce the structural integrity of the tool body in which it is disposed. The antenna embodiment is also relatively wear resistant. The resultant magnetic dipole generated by this antenna is also electrically steerable in inclination angle from a common origin. A variable dipole moment inclination angle combined with independently measured tool rotation orientation during normal drilling allows the antenna to generate a magnetic dipole moment that may be directed at any three dimensional angle and from a common origin point at the centroid of the antenna. The antenna can also be embodied to be more sensitive to resitivity in a particular azimuthal direction.

This continuation-in-part application claims priority to U.S. patentapplication Ser. No. 12/575,566, entitled “Steerable Magnetic DipoleAntenna For Measurement-While-Drilling Applications,” filed Oct. 8, 2009and which is hereby incorporated by reference.

FIELD OF THE INVENTION

This disclosure is directed toward a robust, steerable, magnetic dipoleantenna for Measurement-While-Drilling (MWD) or Logging-While-Drilling(LWD) applications.

BACKGROUND OF THE INVENTION

A measurement of electromagnetic (EM) properties of earth formationpenetrated by a borehole has been used for decades in hydrocarbonexploration and production operations. The resistivity of hydrocarbon isgreater than saline water. A measure of formation resistivity can,therefore, be used to delineate hydrocarbon bearing formations fromsaline water bearing formations. Electromagnetic borehole measurementsare also used to determine a wide range of geophysical parameters ofinterest including the location of bed boundaries, the dip of formationsintersecting the borehole, and anisotropy of material intersected by theborehole. Electromagnetic measurements are also used to “steer” thedrilling of the borehole.

Borehole instruments, or borehole “tools”, used to obtain EMmeasurements typically comprise one or more antennas or transmittingcoils which are energized by an alternating electrical current.Resulting EM energy interacts with the surrounding formation andborehole environs by propagation or by induction of currents within theborehole environs. One or more receivers respond to this EM energy orcurrent. A single coil or antenna can serve as both a transmitter and areceiver. Parameters of interest, such as those listed above, aredetermined from the response of the one or more receivers. Response ofone or more receivers within the borehole apparatus may be telemeteredto the surface of the earth via conveyance means that include a wirelineor a drill string equipped with a borehole telemetry system; such as mudpulse, sonic or electromagnetic telemetry. Alternately, the response ofone or more receivers can be stored within the borehole tool forsubsequent retrieval at the surface of the earth.

Standard induction and wave propagation EM tools are configured withtransmitter and receiver coils with their magnetic moments aligned withthe major axis of the tool. More recently, induction tools with threeaxis coils and wave propagation MWD or LWD tools with antennas (coils)whose magnetic moments are not aligned with the tool axis are beingproduced and used. These MWD or LWD propagation tools, with antennadipole axes tilted with respect to the tool axis, can locate boundarieswith resistivity differences as a function of tool azimuth. Tools withcoils aligned with the tool axis cannot locate boundaries withresistivity changes as a function of tool azimuthal angle. The azimuthalresistivity response feature of an electromagnetic MWD or LWD tool ismost useful in direction or “geosteering” the drilling direction of awell in a formation of interest. More specifically, the distance anddirection from the tool to a bed (such as shale) bounding the formationof interest, or water interfaces within the formation of interest, canbe determined from the azimuthal resistivity response of the tool. Usingthis information, the drill bit can be directed or “steered”, in realtime, to stay within the formation zone of interest so as to avoidpenetrating non hydrocarbon bearing formations with the borehole.

Prior art MWD or LWD tools that make azimuthal EM measurements employ acombination of separate axially aligned antennas and antennas whosemagnetic moments are tilted at an angle with respect to the tool axis.Such tools, for example, are described in U.S. Pat. No. 6,476,609 issuedto Bittar, and U.S. Pat. No. 6,297,639 issued to Clark et al. Thesetools have a fixed inclination and azimuth response, and can onlytransmit or receive magnetic fields at a particular orientation relativeto the tool. These patents include a rotational position sensor and aprocessor to identify the azimuthal angle of the magnetic moments as thetool rotates during drilling. Furthermore, the antennas with differentdipole orientations located at different axial spacings along the lengthof the tool lack a common dipole origin point. This fact precludesvector addition of the dipole moments to form a new dipole moment, inany direction, with the same origin point. Multiple antennas atdiffering axial spacings also increase tool production and maintenancecost, and further reduces mechanical tool strength.

Electromagnetic antennas have been designed for MWD or LWD tools for thepast three decades. The use of highly magnetic permeable material in thedesign of these antennas has been around for the past two decades andantennas that generate a magnetic field in directions other than thetool axis directions have been designed mostly in the past decade. U.S.Pat. No. 4,536,713 issued to Davis et al. describes a high permeabilitymagnetic material disposed in a drill collar used for measuring mudresistivity outside the collar in the annulus region between the drillcollar and the borehole wall. U.S. Pat. No. 5,138,263 issued to Towledescribes placing magnetic material between an antenna wire and an MWDcollar to electromagnetically couple the antenna signal to theformation.

U.S. Pat. No. 6,181,138 issued to Hagiwara describes an arrangement ofthree antennas disposed around a drill collar in which each antenna iscomposed of a coil wire disposed within a plane and oriented at an anglewith respect to the tool axis. Each of the three antennas is basically awire around the outside of a usually steel drill collar, wherein thepath of the wire is located in a plane intersecting the drill collar.The normal vector to this plane can be described as having aninclination angle and an azimuthal angle. Azimuthal angle as it is beingused here is the angle around the tool perpendicular to the tool axis.The origin of the vector is the center of the plane containing theantenna. All of the three antennas have the same centroid or geometriccenter and, as such, produce magnetic vectors that have a common originor are co-located. The patent also describes on the same tool additionalantennas spaced apart along the tool axis and oriented at a second anglewith respect to the tool axis. The additional antennas are disposedwithin a plane that makes an angle of zero degrees in the same mannerthat standard wave propagation resistivity tools are constructed. Thepatent also discloses using the antennas in combination with arotational position sensor and a processor contained within the MWDtool. The patent also describes combining the three antennas toelectrically orient the antenna magnetic dipole moment to any azimuthalangle, but cannot change the inclination angle. This antenna designplaces coils around a drilling collar in a region of reduced diameter or“necked down” region. It is well known in the art that reducing theouter diameter of a drilling collar weakens it in that area and causesthe collar to be more prone to mechanical failure. In this design alsothe coils must be covered with a non-conducting layer which must go allthe way around the collar for the extent of the tilted coils.Non-conductive coverings presently used in the art such as fiberglass,rubber, epoxy, ceramics or plastic are subject to wear due to abrasionwhich occurs between the tool and the borehole wall, and are not asstrong as the collar material. Because the non-conducting region mustencircle the collar it is likely to contact the borehole wall unless thecollar is further “necked down” causing further weakness. An extremepenalty is paid by “necking down” drilling tubulars. It is well known tothose skilled in the art that reducing the outer diameter of acylindrical member reduces the torsional and bending stiffnessproportional to the forth power of the radius. For example, reducing thediameter of a 5 inch (12.7 centimeter) tubular to 4 inches (10.2centimeters) reduces the torsional and bending stiffness by 59%.

U.S. Pat. No. 6,476,609 issued to Bittar describes at least one antennadisposed in a plane and oriented at an angle with respect to the toolaxis and another antenna displaced along the tool axis from the firstantenna and disposed in a plane and oriented in a different angle withrespect to the tool axis. This patent also includes a rotationalposition sensor and a processor.

U.S. Pat. No. 7,038,457 issued to Chen and Barber, and U.S. Pat. No.3,808,520 issued to Runge, describe co-located triaxial antennaconstruction in which three orthogonal coils are wound around a commonpoint on a borehole logging tool. These patents describe the virtues ofhaving antennas with three orthogonal dipole moments all passing throughthe same point in the center of the logging tool. The teachings of bothpatents are more suitable for tools conveyed into a borehole bywireline, rather than tools used in drilling a borehole, because thedisclosed coil windings would compromise the strength and durability ofan MWD or LWD tool. Runge describes a triaxial antenna located in thecenter of a tool with non-conducting tool housing or “mandrel” aroundit. This design is clearly not appropriate for MWD or LWD embodiment. Itis known to those of ordinary skill in the MWD or LWD art that anon-conducting tool body does not have the strength to support thesevere mechanical requirements of tools used in drilling. Chen andBarber describe a technique for implementing an antenna structure withco-located magnetic dipole moments in which the transverse coilspenetrate a mandrel through openings in the tool body. While this may beappropriate for wireline applications, openings in the tool body inwhich a coil is placed will cause weakness in the tool body. In additionprovision must be made for drilling fluid or drilling “mud” to flow downwithin the body of an MWD or LWD tool. This mud usually flows in aconduit or channel in the center of the MWD or LWD tool, which istypically a drill collar. Embodied in a MWD or LWD system, the Chen andBarber design must somehow be modified to divert the mud away from thecoils and the openings in the tool body thereby adding complexity andcost to the manufacture of the tool. Another problem encountered inembodying the Chen and Barber design as an MWD or LWD system is that,owing to the required non-conductive covering which is disposed aroundthe circumference of the tool, the coils are not protected from abrasionwhich occurs between the tool and the borehole wall during drilling.

A more robust antenna design suitable for MWD or LWD application isdescribed in U.S. Pat. No. 5,530,358 issued to Wisler et al. Thisantenna is integrated into a drilling tubular affording maximum strengthand abrasion resistance, One of the key components of the Wisler et al.system is the antenna is composed of grooves and wire pathways disposedbeneath the surface of the drilling tubular surface to avoid anyabrasion and so as not to reduce the strength of the tubular. The patentfurther discloses disposing magnetic material between the wire and thegrooves.

U.S. Pat. No. 7,057,392 issued to Wang et al describes an antenna withgrooves on the outside of the tool that are oriented “substantiallyorthogonal to the tool axis”. The antenna construction and grooves aresimilar to those described in U.S. Pat. No. 5,530,358.

SUMMARY OF THE INVENTION

The present invention describes a robust, steerable, magnetic dipoleantenna for Measurement-While-Drilling (MWD) or Logging-While-Drilling(LWD) applications. The antenna elements use a hole arrangement inaddition to grooves in a steel tool body, which is typically a drillcollar. This antenna embodiment produces an extremely robust, antennathat does not significantly reduce the structural integrity of the toolbody in which it is disposed. The antenna embodiment is also wearresistant in harsh MWD or LWD environments, as will be illustrated indetail in subsequent sections of this disclosure. Also the resultantmagnetic dipole generated by this antenna is not generated by a wiredisposed in a single plane as the prior art and is thereby electricallysteerable in inclination angle from a common origin. A variable dipolemoment inclination angle combined with independently measured toolrotation orientation during normal drilling allows the antenna togenerate a magnetic dipole moment that may be directed at any threedimensional angle and from a common origin point at the centroid of theantenna. In the context of this disclosure, “inclination angle” is theangle that the antenna magnetic dipole moment forms with the tool axis.

The antenna elements can also be embodied to exhibit sensitive toresistivity in a particular azimuthal direction. In this embodiment,magnetic fields are preferentially directed thereby inducing largecurrents in the part of earth formation on one particular side of thetool body, while on the opposite side, the currents are much reduced.Secondary magnetic fields caused by the formation currents are detectedby additional antenna(s) resulting in a signal that is dependent on theresistivity on a particular side of the tool. The antenna embodiment iscomposed of at least three magnetic dipole elements, which may besimultaneously energized to produce the preferentially directed magneticfield. A LWD or MWD logging tool using this embodiment may be used todetect distances to formation boundaries of differing resistivities forgeosteering applications. Prior art boundary detecting tools use atilted magnetic dipole that has the same magnitude of field on oppositesides of the tool. As such prior art tools cannot judge distance to bedwhen the tool is midway between two bed boundaries.

As mentioned previously, the antenna elements including antenna wiresare disposed within both recesses or “grooves” and within tunnels or“holes” in the wall of the tool body. In both types of elements thewires are in close proximity with soft ferromagnetic material. Thegrooves are used on the part of the antenna where antenna wire issubstantially perpendicular to the major tool axis. The grooves aresimilar to those described in previously mention U.S. Pat. No.5,530,358, which is herein entered into this disclosure in its entiretyby reference. The holes are used on the part of the antenna whereantenna wire is substantially parallel to the tool axis and in apreferred embodiment the holes are substantially perpendicular to thetool axis. As is well known in the MWD industry, most wear takes placedue to the rotation of the MWD tools in the borehole, and as suchscoring and wear takes place in a plane roughly perpendicular to thetool axis. In general, non-conducting materials used to cover antennaelements are not as resistant to wear and gouging as steel and otherconductors. For this reason, grooves that go around the periphery of thetool body and are covered with non-conducting material are much morelikely to be destroyed than are grooves that are covered innon-conducting material and which are essentially parallel to the toolbody. The present invention uses a more robust hole element designinstead of groove elements for the elements that are essentiallyperpendicular to the tool axis.

The groove antenna elements of this disclosure comprise softferromagnetic material covered by a non-conducting material. The holeelements are filled with soft ferromagnetic material and are oriented atan angle to the tool axis. In a preferred embodiment holes are orientedsubstantially perpendicular to the tool axis.

Although the antenna array is disclosed as being embodied in a MWD orLWD logging system, it should be understood that concepts of the antennacan be applied to any system that rotates within a well borehole.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features and advantages ofthe present invention are obtained and can be understood in detail, amore particular description of the invention, briefly summarized above,may be had by reference to the embodiments thereof which are illustratedin the appended drawings.

FIG. 1 a shows an azimuthal cross section view of a tool housingcomprising antenna grooves;

FIG. 1 b shows a side view of the of the tool housing of FIG. 1 a;

FIG. 2 a is a cross sectional view of the wall of the tool housingillustrating elements of an antenna within two exemplary groovesparallel to the axis of the tool housing;

FIG. 2 b shows conceptually the effect of wear on the grooves shown inFIG. 2 a due to downhole rotation of the tool housing within a borehole;

FIG. 2 c shows an enlarged view of a single worn groove that is parallelto the tool axis;

FIG. 3 a is a cross sectional view of the wall of the tool housingillustrating elements of a prior art antenna groove perpendicular to theaxis of the tool housing;

FIG. 3 b shows conceptually the effect of wear on the prior art grooveshown in FIG. 3 a due to downhole rotation of the tool housing within aborehole;

FIG. 3 c shows a side view of prior art grooves oriented perpendicularto the major axis of the tool housing;

FIG. 4 a is a radial cross sectional view of two antenna holes orientedwith their major axes perpendicular to the tool housing axis;

FIG. 4 b illustrates an axial cross sectional view of the same toolhousing shown in FIG. 4 a comprising a plurality or “set” of axiallyspaced antenna holes;

FIG. 4 c is a side view of an alternate embodiment of the antennaelements showing a set of antenna elements where holes and slits arepositioned at an angle Φ that is not perpendicular to the major axis ofthe tool;

FIG. 4 d is a side view of the alternate embodiment of the antennaelements showing portions of opposing antenna elements where holes andslits are positioned at an angle Φ that is not perpendicular to themajor axis of the tool;

FIG. 5 a illustrates a more detailed view of an antenna holeperpendicular to the axis of the logging tool;

FIG. 5 b illustrates conceptually the results of borehole wear on theantenna hole shown in FIG. 5 a;

FIG. 6 is a side view of the exterior of a MWD logging tool sectionhousing a steerable magnetic dipole antenna;

FIG. 7 is a perspective wiring diagram of antenna wires used in thesteerable magnetic dipole antenna shown in FIG. 6;

FIG. 8 shows the exemplary magnetic moment vectors generated by theantenna that are parallel to the axis of the tool housing;

FIG. 9 shows a flow diagram comprising the major antenna transmissionelements;

FIG. 10 shows magnetic moments M1 and M2 generated by currents I1 and I2in the antenna in addition to the resultant magnetic moment of theantenna MR;

FIG. 11 shows the normalized antenna input currents I1 and I2 requiredto produce a resultant magnetic moment vector MR in any direction withinthe X-Z plane;

FIG. 12 shows a flow diagram comprising the major antenna receiverelements.

FIG. 13 a shows a section view of the antenna elements in a planecontaining the major axis of the tool;

FIG. 13 b shows a section of the “tunnel” of the hole antenna element ina plane whose normal vector is parallel to major axis of the tool;

FIG. 14 a shows conceptually a net surface current flowing on theoutside surface of the tool resulting from the operation of hole antennaelements;

FIG. 14 b shows a magnetic fields caused by surface current and anothersurface current on the other side of the tool going in the oppositedirection.

FIG. 15 is a conceptual illustration of a prior art logging toolcomprising a dipole antenna that is tilted by 90 degrees with respect tothe major axis of the tool;

FIG. 16 is a prior art logging tool comprising a dipole antenna that istilted by 45 degrees with respect to the major axis of the tool;

FIG. 17 is a side view of the exterior of a side-looking MWD antennatool section;

FIG. 18 illustrates the configuration of the single wire element of theside-looking antenna;

FIG. 19 illustrates conceptually the magnetic dipole vectors created ordetected by the side-looking antenna;

FIG. 20 shows current fields generated by side-looking antennacomprising a three dipole array;

FIG. 21 is a conceptual illustration of a prior art tools whoseresponses are in a borehole penetrating a 20 ohm-m formation;

FIG. 22 is a conceptual illustration of a tool comprising a side-lookingantenna disposed in the borehole penetrating a 20 ohm-m formation, and

FIG. 23 conceptually illustrates the orientation of the dipoles in afive dipole section antenna.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes a robust, steerable, magnetic dipoleantenna for 10 kilohertz (kHz) to 10 megahertz (MHz)Measurement-While-Drilling (MWD) or Logging-While-Drilling (LWD)applications. The antenna elements comprise one or more antenna “hole”elements in addition to one or more antenna “groove” elements in a steeltool body, which is typically a drill collar. This embodiment producesan extremely robust antenna that does not significantly reduce thestructural integrity of the tool body in which it is disposed. Theantenna embodiment is also relatively wear resistant to the harsh MWD orLWD environments. For brevity, both MWD and LWD systems will be referredto as “MWD” systems.

Using antenna hole elements perpendicular to the tool axis only, amagnetic field can be generated or received perpendicular to the majoraxis of the tool. Using groove elements parallel to the tool axis only amagnetic vector can be generated or received parallel to the major axisof the tool. Using both hole and groove antenna elements, a magneticfield may be generated or received at any inclination angle. Thisvariable inclination, combined with an independent measure of toolazimuthal orientation during rotation, provides the ability to transmitor receive electromagnetic fields with magnetic vectors in anydirection. The antenna elements can also be embodies to exhibit moresensitivity to resistivity in a particular azimuthal direction. Antennaelement responses can subsequently be used to determine the location ofthe tool and to steer the direction of the MWD system during a drillingoperation.

Operational Wear Patterns in MWD Logging Tools

In order to fully understand the advantages of the present invention, itis instructive to examine operational wear patterns of MWD loggingtools.

FIGS. 1 a and 1 b show antenna recesses or “grooves” configuration usedin U.S. Pat. No. 5,530,358, which has been entered into this disclosureby reference. These grooves are parallel to the major axis of thelogging tool. FIG. 1 a shows an azimuthal cross section view at A-A of atool housing 20 for the steerable dipole antenna section of a MWD tool.Grooves 23 are disposed azimuthally around the outer surface of the toolhousing 20. The azimuthal spacing is preferably equal. As mentionedpreviously, the tool housing 20 is preferably a drill collar comprisinga conduit 22 through which drilling fluid flows. The tool housing 20 isshown disposed within a borehole 33 defined by a borehole wall 28 andpenetrating an earth formation 29. FIG. 1 b shows a side view of the ofthe tool housing 20, and clearly shows a “set” of grooves 23, with eachgroove being essentially parallel to the major axis of the tool housing20. Each groove is, therefore, essentially parallel to the major axis ofthe MWD logging tool. The axial position of each groove in the set ispreferably the same along the tool body 20.

FIG. 2 a is a cross sectional view of the wall of the tool housing 20illustrating elements of the antenna within two exemplary grooves 23from the set of grooves shown in FIGS. 1 a and 1 b. The radially inwardor “bottom” portion of each groove comprises ferromagnetic material 30.The radially outward or “top” of each groove comprises non-conductingmaterial 18. Antenna wire 16 traverses the non-conducting material in adirection that at any point is essentially perpendicular to the majoraxis of the tool housing 20. Antenna wire within the wall of the toolhousing 20 between grooves, disposed in a wireway 13, is indicated bybroken lines. Details of the wireway 13 are disclosed in U.S. patentapplication Publication Ser. No. 11/685,046 filed Mar. 12, 2007 andassigned to the assignee of the present invention, which is entered intothis disclosure in its entirety by reference. FIG. 2 a illustrates thegeometry of the antenna before the tool housing 20 has experienced wearwithin the well borehole.

FIG. 2 b shows conceptually the effect of wear on the grooves 23 shownin FIG. 2 a due to downhole rotation of the tool housing 20 within aborehole. Part of the non-conducting covering 18 has been worn away asillustrated by the curvature of surface 18 a. Some non-conductingmaterial does, however, remain in contact with the ferromagneticmaterial and the antenna wire 16 remains intact. This wear pattern hasno adverse effect on the antenna performance.

FIG. 2 c shows an enlarged view of a single worn groove 23 that isessentially parallel to the tool axis, and with a wear depth 26approximately equal to the groove width 28. As a general rule, it hasbeen found that under normal operating conditions, the radial depth 26of non-conducting material 18 that will be worn away is approximatelyequal to the width 28 of the groove. Based upon this finding, the radialdepth 26 of a groove is made greater than the azimuthal width 28 of thegrooves. For the antenna of this disclosure, dimensions of grooves 23are approximately 0.25 inches (0.64 centimeters) wide and at least twiceas deep. With these relative dimensions, the non-conducting material 18disposed within in the groove will only be worn approximately the same0.25 inches (0.64 centimeters) or less leaving sufficient non-conductingmaterial and the antenna wire to insure normal operation of the antenna.

The wear pattern of grooves oriented perpendicular to the major axis ofthe tool, and therefore oriented in the direction of tool rotation, arenext examined Wear patterns in grooves oriented perpendicular to thetool axis can be catastrophic to the operation of the antenna. FIG. 3 cshows a side view of prior art grooves 23 oriented perpendicular to themajor axis of the tool 20. FIGS. 3 a and 3 b, which are cross sectionalviews of the grooves in FIG. 3 c oriented perpendicular to the toolaxis, are used to illustrate conceptually these wear patterns. Morespecifically, FIG. 3 a shows the ferromagnetic material 30, thenon-conductive covering material 18, and the antenna wire 16 disposedwithin a “azimuthal” groove 25 before downhole use. FIG. 3 b showselements within the same groove 25 after the tool has been rotatedwithin the borehole. The wear contour is illustrated by the surface 18b. Both the non-conducting material 18 and the ferromagnetic material 30have been worn away along with the antenna wire 16 imbedded within thenonconducting material. Stated another way, FIG. 3 b illustrates thecatastrophic effect of wear when the dimension of the groove in thedirection of rotation is much larger than the depth of the groove. Awear pattern illustrated conceptually in FIG. 3 b obviously renders theantenna inoperable.

Holes for Antenna Elements Perpendicular to the Tool Axis

In order to avoid catastrophic wear patterns of antenna elementsoriented perpendicular to the tool axis, a new type of antenna elementis employed. These elements comprise drilled holes filled with ferriteand a thin saw cut or “slit” along the hole length. Within the contextof this disclosure, the term “hole antenna element” refers to a part ofthe tool comprising a tunnel or hole within the wall of the tool whosecenter is a chord in a cylindrical section of the tool, a slit extendingfrom the hole to the outer surface of the tool, the outer surface of thetool in the vicinity of the slit, and an antenna wire element traversingthe hole and located between the hole and the tool outer surface.

FIG. 4 a is a radial cross sectional view at A-A of two hole antennaelements or 31 and 32 oriented with their major axes perpendicular tothe tool housing axis, traversing the wall of the tool housing 20, andazimuthally spaced at 180 degrees. The major axis of each hole is alsopreferably perpendicular to the radius of the tool housing 20. The holes31 and 32 contain ferromagnetic material 30 such as ferrite.Corresponding antenna wires are denoted by 41 and 42, respectively. Theconduit through which drilling fluid flows is again denoted by 22. FIG.4 b illustrates a side view of the same tool housing 20 comprising aplurality or “set” of axially hole antenna elements, the openings of theholes are denoted by 31. Hole antenna elements 32 (see FIG. 4 a) are onthe opposite side of the tool housing 20 and, therefore, are not shownin FIG. 4 b. The thin saw cuts or “slits” which intersect the holesalong their length are denoted by 110. The slits 110 are filled withnon-conductive wear resistant material that will be subsequentlydiscussed in more detail. The axial spacing of the elements in the setis preferably equal.

An alternate embodiment of the antenna hole elements is shown in FIGS. 4c and 4 d in which the holes and slits are positioned at an angle Φ at67 that is not perpendicular to the major axis of the tool 20. In thiscase the hole part of each hole elements, indicated by openings 31 and32, are located along a chord of elliptical conic sections of the toolbody, wherein the planes defining the conic sections are notperpendicular to the tool axis. More specifically, FIG. 4 c illustratesa side view of the set of hole elements 32, and FIG. 4 d is a side viewshowing portions of the sets of opposing hole elements 32 and 32. Thisembodiment will provide a magnetic vector that is not perpendicular tothe major axis of the tool 20, but makes an angle Φ with theperpendicular vector 67 a as shown in FIG. 4 c. In this manner, theantenna generates or detects a field component in the perpendiculardirection 67 a as well as a component in the axial direction of themajor axis of the tool 20. Although not show one familiar with the artof antenna design will realize that the hole elements can have differenttilt angles, Φ, and can be located at different azimuthal locationsrelative to one another in order to achieve alternate embodimentantennas with differing characteristics.

FIG. 5 a illustrates a more detailed view of one end of a single holeelement 31 perpendicular to the axis of the logging tool 20. The hole 31is preferably a round conduit, although other shapes can be used. Theazimuthally spaced hole elements 32 (see FIG. 4 a) are identical to thehole element 31. In the present embodiment of the steerable dipoleantenna system, openings of the holes are essentially round holesapproximately 0.25 inches (0.64 centimeters) in diameter. The holescontain ferromagnetic material 30 and are terminated at each end (onlyone end shown) by non-conducting inserts 37. The ferromagnetic material30 is recessed at least 0.25 inches (0.64 centimeters) from the outersurface of the tool housing 20. The slits 110 (see FIG. 4 b) are verythin and preferably less than 1/16 inch (0.16 cm) wide so that they willnot erode during drilling.

FIG. 5 b illustrates conceptually the results of borehole wear on theends of the hole element shown in FIG. 5 a. The wear of thenon-conducting insert 37 is illustrated by the contour of the surface 37a. As discussed above, if the diameter of the hole is 0.25 inches (0.64centimeters) or less, and the radial length of the non-conducting insertis greater than 0.25 inches (0.64 centimeters), the depth to which theinsert erodes is 0.25 inches (0.64 centimeters) or less, and does notdamage the operation of the antenna.

Configuration of the Steerable Magnetic Dipole Antenna Tool Section

FIG. 6 is a side view of the exterior of a MWD logging tool section 20housing a preferred embodiment of the steerable magnetic dipole antenna.The antenna comprises a first set 36 and a second set 38 of axiallygrooved and axially spaced antenna elements. The grooves in each set 36and 38 are essentially parallel to the major axis of the tool section20, and are azimuthally disposed peripherally around the outer surfaceof the housing 20 (see FIGS. 1 a and 1 b). Sections of antenna wires 40and 42, which are perpendicular to the axis of the tool section 20,traverse each groove set 36 and 38. Sections of antenna wires 40 and 42also traverse tool housing material between grooves within wireways (notshown). Wireways housing antenna wires are described in detail in U.S.patent application Ser. No. 11/685,046 which has been entered into thisdisclosure by reference. Broken lines represent the antenna wiresbeneath the outer surface on the tool 20.

Again referring to FIG. 6, a first set or transversally directed holeantenna elements are shown with hole openings 31. A second set of holeantenna elements with hole openings 32 (see FIG. 4 a) is disposed on theother side of the tool displaced by 180 degrees of azimuth angle and,therefore, not shown in this view. These transverse hole elements (seeFIGS. 4 a and 5 a) are disposed axially between the first and secondsets of axial grooves 36 and 38, respectively. The dotted linesrepresent sections of the antenna wires 40 and 42 disposed innon-conducting material or within in wireways (see FIGS. 2 a-2 c) withinthe wall of the tool section 20. Axial portions of the antenna wires 40and 42, which are portions parallel to the axis of the tool section 20,are disposed within in a common wireway. and are disposed above theferrite in the holes as shown in detailed FIGS. 13 a and 13 b. Slitsbetween the holes are again denoted as 110. The ends of the antennawires 40 and 42 terminate at antenna wire connection boxes 44 and 46,respectively. The antenna wire connection boxes serve as terminalsthrough which the antenna wires 40 and 42 are connected electricallywith power supplies, control electronics, and the telemetry system ofMWD logging tool. This connection is indicated conceptually as “To 70”,and will be subsequently discussed in more detail.

FIG. 7 is a perspective wiring diagram of the antenna wires 40 and 42that traverse the first and second sets of groove antenna elements 36and 38, respectively, and over the hole elements indicated by openings31 and 32. (see FIGS. 13 a and 13 b for details)

The leads 40 a and 42 a connect the antenna wires 40 and 42, via theconnection boxes 44 and 46, respectively, to transmission receivercircuit 70.

Circuit and Operation of the Steerable Magnetic Dipole Antenna

Conceptual Overview

Attention is directed to FIGS. 7 and 8. Currents I1 and I2 are suppliedto, or generate by, the antenna wires 40 and 42 at wire connection boxes44 and 46, respectively, depending upon whether the antenna is operatingas a transmitter or receiver. An understanding of how the steerablemagnetic dipole antenna operates can be seen by assuming that theantenna is in a transmission mode. The currents I1 and I2 are adjustedto obtain said magnetic moment vector in a predetermined direction. Whenthe axial elements 36 and 38 are activated, there results a magneticvector in the Z direction, where Z is coincident with the major axis ofthe tool section 20 as indicated at 50. When the transverse elements 31and 32 are activated, there results a magnetic vector in the Xdirection, where X is perpendicular to the major axis of the toolsection 20. The antenna may be used as a transmitter or as a receiver.

An understanding of how the steerable magnetic dipole antenna operatescan be seen by assuming that the antenna is operating in a transmissionmode. Using the transmitter-receiver circuit 70, the currents I1 and I2are therefore controlled to direct the magnetic vector from the Zdirection, or from the X direction, or all inclination angles therebetween in the X-Z plane. As an example, when I1=I2 in the wiringdiagram shown in FIG. 7, the net current in the axially directed wires40 and 42 is zero and there is, therefore, no net current to activatethe transverse hole element sets 31 and 32. The net currents go aroundthe periphery of the tool section 20 in the sections of wires 40 and 42that activate the axially grooved antenna element sets 36 and 38. Thenet result is a magnetic vector directed in the Z or “axial” direction.When the currents are controlled such that I1=−I2, the currents in thetransverse elements 31 and 32 are 2×I1, and these elements are activatedto produce a magnetic vector in the X direction. Also when I1=−I2,currents in the antenna wires activating the axial groove elements 36and 38 produce field vectors which cancel.

By reference to FIG. 8, the resulting magnetic vectors are shown. Twovectors 60 and 62 are in the plus Z direction and two vectors 64 and 66are in the minus Z direction, with the net magnetic dipole moment beingzero. The inclination angle may be adjusted by varying the I1 and I2currents, as will be detailed in the following section. Note that theslits are again denoted as 110.

Circuit and Operation Details

FIG. 9 shows a flow diagram comprising the major antenna transmissionelements 70 a of the transmitter-receiver circuit 70. Operating as atransmitter, a processor 72 sends data to two oscillator basedtransmitter circuits 74 and 76, which create two antenna input signals.The oscillators are numerically controlled to adjust the input signalsto specified frequency, phase, and amplitude. These input signals arethe previously discussed antenna input currents I1 and I2, which areindicated conceptually at 78 and 79, respectively. The currents I1 andI2 are input to the antenna at the same frequency via the antenna wireconnection boxes 44 and 46. The phase and amplitude of each individualantenna input is adjusted via the numerically controlled oscillators 74and 76 to change the amplitude and inclination angle of the resultantantenna magnetic moment, which is a combination of the magnetic momentsgenerated by the individual antenna current inputs. This process isstated mathematically below. Preferably mathematical computations areperformed in the processor 72. It is also noted that some elements inthe transmitter or receiver circuit 70 (See FIGS. 7 and 9) are used whenthe antenna is operated as a receiver, as will be subsequentlydiscussed. An example of such an element is the processor 72,

FIG. 10 is a graphical illustration of the component and resultantmagnetic moments. The component magnetic moments M1 at 92 and M2 at 94are generated by currents I1 and I2 in the antenna, which is operatingas a transmitter. The resultant magnetic moment of the antenna MR at 96is the vector sum of the two components M1 and M2. Statedmathematically:MR=M1+M2The angles θ1 at 91 and θ2 at 93 are the inclination angles that thecomponent magnetic vectors M1 and M2, respectively, make with the toolaxis, and may be represented by:

${M\; 1} = {I\;{1 \cdot {C\begin{pmatrix}{\cos\left( {\theta\; 1} \right)} \\{\sin\left( {\theta\; 1} \right)}\end{pmatrix}}}}$ and ${M\; 2} = {I\;{2 \cdot {C\begin{pmatrix}{\cos\left( {\theta\; 2} \right)} \\{\sin\left( {\theta\; 2} \right)}\end{pmatrix}}}}$C represents a conversion constant from current to magnetic moments forboth the individual components. C, and the angles θ1 and θ2, aremeasured constants that are functions of the antenna structure and donot change. The resultant vector is therefore:

${MR} = {C\begin{pmatrix}{{I\;{1 \cdot {\cos\left( {\theta\; 1} \right)}}} + {I\;{2 \cdot {\cos\left( {\theta\; 2} \right)}}}} \\{{I\;{1 \cdot {\sin\left( {\theta\; 1} \right)}}} + {I\;{2 \cdot {\sin\left( {\theta\; 2} \right)}}}}\end{pmatrix}}$where all terms on the right hand side of the equation are known or canbe determined.If it is required that the resultant magnetic moment MR be a constant‘K’ independent of θ1, θ2, I1 and I2|MR|=Kor[(I1·cos(θ1)+I2·cos(θ2))·C] ²+[(I1·sin(θ1)+I2·sin(θ2))·C] ² =K ².The tangent of the inclination angle θR between the tool axis (Z axis)and the resultant magnetic vector MR is:θR=atan2(I1·sin(θ1)+I2·sin(θ2),I1·cos(θ1)+I2·cos(θ2))where “atan2” is the standard Fortran or C computer language notationfor the arctangent function that has principal values from 180 degreesto −180 degrees. Given the known constants θ1 and θ2 and the requiredmagnetic moment of the resultant K, the last two equations may be solvedfor I1 and I2 by standard, well known methods for any given steeringangle θR. In particular if the antenna is constructed such thatθ1=θ2=45 degreesThe above equations reduce to:θR=atan2(I2−I1,I1+I2)and(I1² +I2²)·C ² =K ²

FIG. 11 shows the normalized antenna input currents I1 and I2 requiredto produce a resultant magnetic moment vector MR in any direction withinthe X-Z plane. More specifically, FIG. 11 shows a plot of antennacurrent inputs (ordinate) as a function of resultant moment direction θR(abscissa), with curve 98 representing input I1 and curve 99representing input current I2. As an example, if I1=−0.707 amps andI2=+0.707 amps, θR=+90 degrees.

Previous discussion has been directed to the antenna operating as atransmitter. FIG. 12 shows a flow diagram comprising the major antennareceiver elements 70 b of the transmitter-receiver circuit 70. When theoperated as a receiver, the physical elements of the antenna areidentical to the antenna operating as a transmitter. Input signals 88and 89 from the antenna wires 40 and 42 (see FIGS. 6 and 7) are inputvia the antenna wire connection boxes 44 and 46 to a first analog todigital (A/D) circuit 82 and a second analog to digital (A/D) circuit84, respectively. The A/D circuits 82 and 84 condition the respectiveinput signals and then convert these signals to digital form. Thedigitized signals are input into the processor 72, which is preferablythe same processor that is used in the transmitter portion (see FIG. 9)of the transmitter-receiver circuit 70. The processor 72 may preferablybe a digital signal processor (DSP). The input signals are thenprocessed and the phase and amplitude of each signal is computed. Thetwo signals are then combined, preferably in the processor 72, toproduce a single signal, which reacts only to a magnetic vector in aparticular inclination angle direction in the X-Z plane. Results canthen be stored in downhole memory at 90 a or telemetered to the surfaceof the earth via a real time MWD telemetry system 90 b. Alternately,measured or “raw” data may be stored in the downhole memory at 90 a ortelemetered to the surface of the earth for subsequent processing. Bothmethods of data storage and transmission are known in the art, anddisclosed in previous disclosures entered into this disclosure byreference. In addition an orientation module 85, which senses theazimuthal angle that the antenna makes with the vertical or the “highside” of the borehole, is simultaneously input to the receiver computer.The orientation data are combined with the received signal data andplaced into bins, wherein each bin contains received signal datareceived when the X-axis of the antenna is in a particular azimuthaldirection. In this way the azimuthal orientation of the antenna data areknown and the received data can be stored, transmitted, or processed asa function of azimuth. The orientation module may be composed of a 3axis magnetometer and/or an inclinometer to sense high side of the holerelative to the earth coordinate system and electronics to relay thisinformation to the receiver computer.

The procedure of data processing with the antenna operating as areceiver is the reverse of the procedure used in operating the antennaas a transmitter, and need not be detailed here. Briefly, the methodinvolves combining the two signals from antenna input 1 and antennainput 2 in the proportion shown in FIG. 11. As an example, if themagnetic field vector to be measured is axially directed (Z direction),the signals from antenna input 1 and antenna input 2 shown at 88 and 89,respectively, would be processed by adding the two signals in phase toproduce an output which is sensitive to the Z component of the fieldexciting the antenna.

Details of Hole Antenna Elements

Details of two hole antenna elements are shown in FIG. 13 a and FIG. 13b. FIG. 13 a shows a section view of the antenna elements in a planecontaining the major axis of the tool 20, and FIG. 13 b shows a sectionof the hole antenna element in a plane whose normal vector is parallelto major axis of the tool 20. The hole part of the hole antenna elementis denoted by 31. Illustrations for hole antenna elements located 180degrees of azimuth or on the other side of the tool are not shown butwould be essentially the same. The drilling fluid conduit is againidentified at 22. Referring first to FIG. 13 a, preferably round holes31 are drilled in a chord (i.e. a line segment connecting two points ona circle whose axis is coincident with the tool axis) and a ferrite rod30 is centered within each hole. Referring to FIG. 13 b as well as FIG.5 a, non-conducting inserts 37, preferably made of a hard plastic suchas PEEK, are placed on either end to secure the rods 30 in place.Referring again to 13 a, it can be seen that sections of antenna wires40 and 42 are disposed in channels 105 below the exterior surface of thetool 20 perpendicular to the hole elements. The previously discussedthin slit 110 extends radially outward from each hole 31,perpendicularly bisects the antenna “feed” wire channel 105, andcontinues radially outward to the surface of the tool 20. This slit 110is made very small and filled with non-conducting wear resistantmaterial such as PEEK or a hard epoxy depicted at 111. The small widthof the slit, less than 1/16 inch (0.16 centimeters), prevents erosion toa depth which will interfere with the operation of the antenna. A netcurrent is shown flowing in antenna wires 40, 42 from left to right byarrows. Within the context of this disclosure, the term “hole antennaelement” refers to a part of the tool comprising a tunnel or hole 31within the wall of the tool whose center is a chord in a cylindricalsection of the tool, containing ferrite rod 30, a slit 110 extendingfrom the hole to the outer surface of the tool, the outer surface of thetool in the vicinity of the slit, and an antenna wire element 40,42.Although two hole antenna elements are shown, the hole antenna arrayantenna may have additional elements depending on the required antennagain. A feature of this invention is that the gain may be increased byenlarging the hole 31 and ferrite 30 diameter without changing otherdimensions in the element.

The general operation of the hole array antenna creates or detects alocalized surface current flowing axially on the tool exterior. By usingsurface currents rather than loop antennas which require a groove theouter surface of the tool 20, the present invention is much more robust.In operation as a transmitter a current is fed onto the antenna wire asshown by the arrow on the antenna feed wires 40 and 42. In reaction tothis current, a secondary surface current, illustrated conceptually withthe arrows 104, is induced into the channel 105 around the wire oppositein direction and nearly equal in magnitude. For a antenna frequenciesgreater than 100 kHz and for the conductivity of the steel tool body 20greater than 1×10⁶ Siemens per meter, the skin depth of the surfacecurrent will be less than 0.06 inch (0.16 centimeters). When the surfacecurrents get to a slit, they must either go toward the surface of thetool 20 or toward the hole 31. The two paths offer differing impedancesto the current flow. The path of the current 104 that goes to thesurface of the tool is constrained to a rectangular path betweenadjacent antenna slits 110 as shown in FIG. 13 a. The path of thecurrent that goes toward the hole 31 must go down one side of the slit110, loop around the ferrite rod 30, travel back up the opposite side ofthe slit 110 toward the outer surface of the tool 20, and continue alongthe wire pathway of the feed wires 40, 42. The current 102 which goesdown to the hole is limited by the impedance of the loop around theferrite rod 30 due to the inductance caused by the ferrite rod. Thecurrent which goes to the surface of the tool 20 is only limited by theimpedance of the surface current in the rectangle. The surface currentrectangle impedance is much less than the path around the ferrite rod,and so most of the current flows to the surface of the tool. When on thesurface of the tool, the current 104 flows in an axial direction alongthe tool body in the same direction as the current in the antenna feedwire 40, 42. This surface current 104 produces magnetic field lines 100shown in FIG. 13 b which are perpendicular to the tool axis at thesurface of the tool. The antenna design of the present invention doesnot require open grooves, but functions with elements comprising a hole31 (or 32) and a small slit 110 in the surface of the tool 20. In thepresent invention, the elements are not a series of separate antennasbut are interconnected by the surface currents to form a single antenna.

The way in which the elements act together to generate a magnetic fieldvector can be seen by reference to FIGS. 14 a and 14 b, which illustratehole antenna elements operating as a transmitter. FIG. 14 a is a sideview of the tool 20 and shows conceptually at 112 a net surface currentflowing on the outside surface of the tool. There is another surfacecurrent flowing in opposite direction on the opposite surface of thetool 20 (see FIG. 14 b). The current has been induced by the feed wire40 or 42 shown in previous drawings. FIG. 14 b is an end view of thetool 20 depicted in a borehole 33 penetrating formation 29. FIG. 14 billustrating conceptually the surface current 112 (shown in FIG. 14 a)flowing “out” of the page, and further illustrates an opposing surfacecurrent 122 flowing “into” the page. Surface current 112 causes a fieldvector indicated by 114. Surface current 122 causes the field vectorindicated at 124. The field current vectors 114 and 124 add together toyield a net vector that is perpendicular to the tool body and whoseorigin the geometric center of the tool.

It should be understood that an antenna can be made of hole elementsalone without acting in concert with groove elements. In this case theantenna magnetic moment is not steerable in the XZ plane but has aconstant azimuth angle other than zero. It should also be understoodthat the circuitry shown in FIGS. 9 and 12 can be used to operate one ormore hole antenna elements. More specifically, the oscillator 74 or 76shown in FIG. 9 can be used to apply a current to antenna wire 40 or 42if the antenna is operated as a transmitter. Furthermore, the oscillator74 or 76 is numerically controlled for adjusting said current togenerate a magnetic field vector perpendicular to said major axis of thetool 20. If the antenna is operated as a receiver, the A/D converters 90a or 90 b shown in FIG. 12 can be used to quantify current induced inthe antenna by a magnetic field vector perpendicular to said major axisof said tool.

The “Side Looking” Antenna Embodiment

As mentioned previously, the antenna can be embodied to exhibitpreferential resistivity sensitivity in a particular azimuthaldirection. This embodiment will be referred to as the “side-looking”antenna. Preferentially directed magnetic fields induce large currentsin formation penetrated by the tool on one particular side of the tool,while on the opposite side of the tool, the currents are much reduced.Secondary magnetic fields caused by the formation currents are detectedby additional antenna(s) resulting in a signal that is dependent on theresistivity on a particular side of the tool. The side-looking antennais composed of at least three magnetic dipole sections that may besimultaneously energized to produce the preferentially directed magneticfield. While is preferable to simultaneously energize the at least threemagnetic dipole sections using common wiring to ensure the phaserelationship among the dipole sections, the sections may beindependently energized by separate circuits simultaneously, or byseparate circuits sequentially. In the case of sequentially energizingthe magnetic dipole sections, measurements of secondary fields byadditional antenna(s) may be stored in a computer memory and combinedmathematically using the well known principal of superposition.

A tool comprising the side-looking antenna can be used to detectdistances to formation boundaries of differing resistivities. Thisinformation can, in turn, be used for geosteering applications. Priorart boundary detecting tools use a tilted magnetic dipole that has thesame magnitude of field, but of opposite phase, on opposite sides of thetool. As such, and as is well known to those skilled in the art, priorart tools cannot judge distance to bed when the tool is midway betweentwo beds of equal resistivity, because the signal from one side cancelsthe signal from the opposite side.

FIG. 15 is a conceptual illustration of a prior art logging tool 215comprising a dipole antenna that is tilted by 90 degrees with respect tothe major axis of the tool. The dipole antenna generates a magneticfield represented at 217. The contours 219, 221 and 223 representlogarithms of current fields of progressively smaller magnitude,respectively. These current fields are induced in a conductive formation229 by the magnetic dipole antenna 217. The ordinate is distance ininches into the formation 229 and the abscissa is distance in inchesalong the major axis of the logging tool 215. It can be seen the log ofthe magnitude of the current field is symmetric with a plane coincidentwith the major axis of the tool 215 and which contains a vectorperpendicular to the plane of the drawing. As is also well known, thephase of the currents in the formation above the tool in FIG. 15 areopposite (180 degrees difference) the phase of the currents in theformation below the tool. When the tool is in a bed midway between twoadjacent beds, the secondary signals from the currents above the tooland below the tool cancel and the distance to either bed cannot bedetermined

FIG. 16 is also prior art and is a conceptual illustration of a loggingtool 215 comprising a dipole antenna that is tilted by 45 degrees withrespect to the major axis of the tool. The dipole antenna generates amagnetic field again represented at 217. The contours 219, 221 and 223again represent logarithms of current fields of progressively smallermagnitude, respectively. These current fields are induced in aconductive formation 229 by the magnetic field 217 and are the samerespective magnitudes as those shown in FIG. 15. Once again, theordinate is distance in inches into the formation 229 and the abscissais distance in inches along the major axis of the logging tool 215.

In the examples shown in both FIGS. 15 and 16, it can be seen that themagnitude of the current field has equal extent up or down. The currentfield would therefore induce the same current but opposite polarity in aformation boundary that is parallel to the tool 215 and at the sameperpendicular distance either above or below the tool. Again it is notbe possible, therefore, to determine the distance of either boundaryfrom the tool since the secondary signals caused by the formationcurrents will cancel.

FIG. 17 is a side view of the exterior of a side-looking antenna housingsuch as a MWD antenna tool 20 section. The mechanical side-lookingantenna configuration is essentially the same as the embodiments shownin FIGS. 6 and 8. Again, the antenna comprises a first set 36 and asecond set 38 of axially grooved and axially spaced antenna elements.The grooves in each set 36 and 38 are essentially parallel to the majoraxis of the tool section 20, and are azimuthally disposed peripherallyaround the outer surface of the housing 20 (see FIGS. 1 a and 1 b). Afirst set or transversally directed hole antenna elements are shown withhole openings 31. A second set of hole antenna elements with holeopenings 32 (see FIG. 4 a) is disposed on the other side of the tooldisplaced by 180 degrees of azimuth angle and, therefore, not shown inthis view. These transverse hole elements (see FIGS. 4 a and 5 a) aredisposed axially between the first and second sets of axial grooves 36and 38, respectively. Slits between the holes are again denoted as 110.

Wiring of the side-looking antenna is significantly different fromprevious embodiments. Referring to both FIGS. 17 and 18, an antenna wire(or a plurality of wires) 240 begins in an impedance matching network210 disposed in the tool 20. For purposes of discussion, it will beassumed that 240 represents a single wire. The impedance matchingnetwork is connected on one end to the transmitter or receiver circuitry(not shown in FIG. 17 or 18) and to the antenna wire (or wires) on theother end. As is well known, the purpose of the matching network is totransform the low impedance antenna circuit comprising a wire or wiresto higher impedance levels that are best handled by the transmitter orreceiver circuitry. The wire 240 perpendicularly traverses one half ofthe groove antenna elements 36 on one side of the tool. The wire 240 isthen disposed axially and subsequently back up the periphery of the tool20 where it then traverses perpendicularly the hole elements 31. Thewire 240 is then disposed peripherally downward, then axially, andsubsequently perpendicularly around the groove elements 38. Next, thewire 240 is disposed peripherally downward, then axially, thenperipherally upward, and subsequently traverses perpendicularly the holeelements 32 (see FIG. 4 a) on the opposite side of the tool 20. In apath illustrated in FIGS. 17 and 18 that is a path essentially describedpreviously, the wire 240 then traverses perpendicularly the remaininggroove antenna elements 36 and the hole elements on the other side ofthe tool, and returns to the impedance matching network 210. Wirewaysand details of the disposition of wire in the wall of the tool 20 andthrough various antenna elements have been previously been disclosed indetail. The output of the impedance matching network 210 goes to eithertransmitter or receiver electronics within the transmitter-receivercircuit 70, depending on whether the antenna is used as a transmitter oras a receiver. Elements of the transmitter-receiver circuit 70 have beenpreviously disclosed in detail.

FIG. 19 illustrates conceptually the magnetic dipole vectors 220, 222and 224 created or detected by the side-looking antenna depicted inFIGS. 17 and 18. The dipole vectors combine to direct a magnetic fieldto one side of the tool antenna housing 20 while essentially cancelingthe field on the other side of the tool.

FIG. 20 shows current fields generated by side-looking antennacomprising a three dipole array. The dipole antenna generates a magneticfield comprising the three previously described components 220, 222, and224. The contours 219, 221 and 223 once again represent logarithms ofthe magnitudes of the current fields of progressively smaller magnitude,respectively, and are the same respective magnitudes as those shown inFIGS. 15 and 16. Once again, the ordinate is distance in inches into theformation 229 and the abscissa is distance in inches along the majoraxis of the logging tool 20. As can be seen by observing the contourlines, the current field is clearly stronger on one side of the tool 20(referred to as above for the purpose of this description) than theother side of the tool (referred to as below for the purpose of thisdescription). If one compares the contour line 221, it can be seen thatresponse of the prior art tools illustrated in FIGS. 15 and 16 “see”about 50 inches (127 centimeters) above and below the tool. Observingthe contour line 221 generated by the three array side-looking antennaresponse shown in FIG. 20, it can be seen that the side-looking antenna“sees” about 60 inches (154 centimeters) above the tool 20 and onlyabout 30 inches (76 centimeters) below the tool. In operation, theseinduced currents create a secondary magnetic field that is picked up byanother suitable receiver antenna. The output of the receiver antenna istherefore most sensitive to the regions that have the highest current.In the prior art systems discussed above and illustrated in FIGS. 15 and16, the sensitivities are equal in magnitude above and below the toolwhereas in the side-looking antenna embodiment, the sensitivity is moreon one side of the tool 20 than the other.

FIG. 21 is a conceptual illustration of a prior art tool 215 (whoseresponse is illustrated in FIGS. 15 and 16) disposed in a borehole 33penetrating a 20 ohm-m formation 262. The formation 262 is bounded topand bottom by 1 ohm-m formations 260 a and 260 b, respectively. Theformation interfaces are shown essentially parallel to the major axis ofthe borehole 33. The borehole is a distance 272 and 274 from the top andbottom formations 260 a and 260 b, respectively. Contour lines,discussed in detail above, are shown in abbreviated form at 270 a and270 b above and below the tool 215. In a typical geosteering example,one might want the borehole 33 to be at the top of the 20 ohm-m zone262. If the distances 272 and 274 are essentially equal, it can be seenin FIG. 21 that the response of prior art tool 215 cannot be used todetermine the position of the borehole 33 with respect to the upperformation 260 a (or the lower formation 260 b). The response of theprior art tool 215 does not, therefore, provide distance informationneeded to geosteer the borehole toward the boundary of the formations262 and 260 a.

FIG. 22 is a conceptual illustration of a tool 20 comprising aside-looking antenna again disposed in the borehole 33 that penetrates a20 ohm-m formation 262. Once again, the formation 262 is bounded top andbottom by 1 ohm-m formations 260 a and 260 b, respectively, and theformation interfaces are shown essentially parallel to the major axis ofthe borehole 33. The borehole 33 is again shown at a distances 272 and274 from the top and bottom formations 260 a and 260 b, respectively.Contour lines generated by the side-looking antenna are shown inabbreviated form at 272 a and 272 b above and below the tool 20. Assumeagain that the objective of the drilling operation is to geosteer theborehole 33 toward the interface of the formations 262 and 260 a. Evenif the distances 272 and 274 are essentially equal, it can be seen inFIG. 22 that the response of the tool comprising the side-looking can beused to determine the position of the borehole 33 with respect to theupper formation 260 a. This is because the response is more sensitive onone side of the tool 20 and therefore provides information needed togeosteer the borehole toward the boundary of the formations 262 and 260a upper bed.

It is noted that the three dipole array antenna described in thisembodiment is based on the principal of the Halbach dipole array, and isthereby not limited to three magnetic antenna dipole sections, but couldbe any number of sections. As an example, the orientation of the dipoles280, 282, 284, 286 and 288 in a five dipole section tool antenna housing20 is shown in FIG. 23.

The principle of operation of Halbach arrays, which concentrates amagnetic field on one side of an array of permanent magnets, can befound in the publication: Halback, J. K. “Design of Permanent MultipoleMagnets with Oriented Rare Earth Cobalt Material”, Nuclear Instrumentsand Methods, vol 169, no. 1, 1980, pp. 1-10. The implementation detailedin this disclosure involves induction or RF frequency dipoles instead ofpermanent magnets but the basic principle of operation is similar.Another implementation of a Halbach array for oilfield application isdisclosed in U.S. Pat. No. 7,541,813 to Harold L. Snyder et al. Theobjective of this disclosure is the elimination of the effect of theconductivity of a steel tool body on the generation of a magnetic field,by externally guiding the magnetic field away from the steel body, anddoes not disclose or suggest geosteering embodiments. Yet anotherreference involving a Halbach array is U.S. Pat. No. 7,853,085 issued toDavid R. Hall et al. This patent also uses the Halbach principle toeliminate the currents in a drill string by focusing the magnetic fieldaway from the conductive drill string, using small arrays mounted on theside of a drilling tool. This patent also does not disclose or suggestgeosteering embodiments. In the present invention, the Halbach principleis used to focus magnetic fields into earth formation on one side of atool (conveyed by a drill string) and to eliminate or reduce fields onthe opposite side of the tool. As such, the present invention uses anarray of larger dipole antennas sections and does not attempt toeliminate the currents in the drill string by using the Halbachprinciple.

SUMMARY

The disclosure sets forth a robust, steerable dipole antenna suitablefor MWD. Novel, robust, hole antenna elements have been disclosed whichfacilitate this end. The antenna can be operated as a transmitter or areceiver. Currents to the antenna array can be adjusted to obtain thedesired antenna magnetic moment inclination vector. Using antenna holeelements only, a magnetic field vector is generated or receivedperpendicular to the major axis of the tool. Using both hole and grooveantenna elements, inclination vector is generated or received in an X-Yplane as define in the disclosure. This inclination vector, combinedwith an independent measure of tool azimuthal orientation duringrotation, yields a measure of tool orientation in three dimensions. Thismeasurement can subsequently be used to steer the direction of the MWDsystem during a drilling operation.

The antenna elements can also be embodied to exhibit sensitive toresistivity in a particular azimuthal direction. In this embodiment,magnetic fields are preferentially directed thereby inducing largecurrents in the part of earth formation on one particular side of thetool body, while on the opposite side, the currents are much reduced.Secondary magnetic fields caused by the formation currents are detectedby additional antenna(s) resulting in a signal that is dependent on theresistivity on a particular side of the tool. This tool response canalso be used to steer the direction of the MWD system during a drillingoperation.

While the foregoing disclosure is directed toward the preferredembodiments of the invention, the scope of the invention is defined bythe claims, which follow.

What is claimed is:
 1. An electromagnetic antenna including an antennawire and an array of magnetic dipole antenna elements disposed withinantenna housing wherein said dipole elements are configured to direct amagnetic field to one side of said antenna housing while essentiallycanceling said magnetic field on the opposite side of said antennahousing, said antenna further comprising: a first and a second set ofgrooved antenna elements wherein said first and said second sets ofgrooved antenna elements are axially spaced along said antenna housing,grooves comprising each said set of grooved antenna elements areessentially parallel to the major axis of said antenna housing, and saidantenna wire perpendicularly traverses said grooves perpendicular tosaid major axis; and a first and a second set of hole antenna elementsdisposed axially in said antenna housing between said first and secondsets of grooved antenna elements, wherein said first and said secondsets of said hole antenna elements are azimuthally spaced around theperiphery of said antenna housing, and holes comprising each said set ofhole antenna elements are essentially perpendicular to said major axisof said antenna housing.
 2. The antenna of claim 1 wherein: said holeantenna elements comprise ferromagnetic material disposed within each ofsaid holes; non-conducting inserts terminates each end of said holes; aslit extends radially outward from said hole to an outer surface of saidantenna housing; and said antenna wire perpendicularly traverses eachsaid slit parallel to said major axis.
 3. The antenna of claim 2 whereinsaid slit is filled with non-conducting wear resistant material.
 4. Amethod for creating or detecting a magnetic field by disposing within anantenna housing an electromagnetic antenna comprising an array of dipoleantenna elements and connecting said antenna elements by means of anantenna wire and configuring said dipole antenna elements to direct saidmagnetic field to one side of said antenna housing while essentiallycanceling said magnetic field on the opposite side of said antennahousing, said method comprising: providing a first and a second set ofgrooved antenna elements wherein said first and said second sets ofgrooved antenna elements are axially spaced along said antenna housing,grooves comprising each said set of grooved antenna elements areessentially parallel to the major axis of said antenna housing, and saidantenna wire perpendicularly traverses said grooves perpendicular tosaid major axis; and providing a first and a second set of hole antennaelements disposed axially in said antenna housing between said first andsecond sets of grooved antenna elements wherein said first and saidsecond sets of said hole elements are azimuthally spaced around theperiphery of said antenna housing, and holes comprising each said set ofhole elements are essentially perpendicular to said major axis of saidantenna housing; and energizing said antenna wire to direct saidmagnetic field to one side of said tool while essentially canceling saidmagnetic field on the opposite side of said tool.
 5. The method of claim4 further comprising: disposing ferromagnetic material within each ofsaid holes of said hole antenna elements; terminating each end of saidholes with non-conducting inserts; forming a slit extending radiallyoutward from said hole to an outer surface of said antenna housing; andperpendicularly traversing each said slit parallel to said major axiswith said antenna wire.
 6. The method of claim 5 further comprisingfilling said slit with non-conducting wear resistant material.
 7. Ameasurement-while-drilling tool comprising a magnetic dipole antennaarray operationally connected by an antenna wire to direct a magneticfield to one side of a drill collar while essentially canceling saidfield on the opposite side of said drill collar, said tool furthercomprises: a transmission-receiver circuit; wherein said magnetic dipoleantenna array comprises a first and a second set of grooved antennaelements wherein said first and said second sets of grooved antennaelements are axially spaced along said drill collar, and groovescomprising each said set of grooved antenna elements are essentiallyparallel to the major axis of said drill collar, and an antenna wirethat perpendicularly traverses said grooves perpendicular to said majoraxis; and a first and a second set of hole antenna elements disposedaxially in said drill collar between said first and second sets ofgrooved antenna elements wherein said first and said second sets of saidhole elements are azimuthally spaced around the periphery of said drillcollar, and holes comprising each said set of hole elements areessentially perpendicular to said major axis of said drill collar. 8.The measurement-while-drilling tool of claim 7 wherein: said holeantenna elements comprise ferromagnetic material disposed within each ofsaid holes; non-conducting inserts terminate each end of said holes; aslit extends radially outward from said hole to an outer surface of saidantenna housing; and said antenna wire perpendicularly traverses eachsaid slit parallel to said major axis of said tool.
 9. Themeasurement-while-drilling tool of claim 8 wherein the radial dimensionof each said insert is greater than the azimuthal dimension of saidinsert.
 10. The measurement-while-drilling tool of claim 7 wherein saidgrooves comprise: ferromagnetic material disposed in the radially inwardportion of each of said grooves; non-conducting material disposed withinthe radially outward portion of each of said grooves; and said antennawire perpendicularly traverses said non-conducting material.
 11. Amethod for steering the drilling of a borehole by creating or detectinga magnetic field on one side of a MWD tool while essentially cancelingsaid field on the opposite side of said tool, said method comprising:providing a first and a second set of grooved antenna elements whereinsaid first and said second sets of grooved antenna elements are axiallyspaced along said tool, grooves comprising each said set of groovedantenna elements are essentially parallel to the major axis of saidtool; providing a first and a second set of hole antenna elementsdisposed axially in said antenna housing between said first and secondsets of grooved antenna elements wherein said first and said second setsof said hole elements are azimuthally spaced around the periphery ofsaid tool, and holes comprising each said set of hole elements areessentially perpendicular to said major axis of said tool; providing anantenna wire; operationally connecting said antenna elements by means ofsaid antenna wire; and energizing said antenna wire to direct a magneticfield to one side of said tool while essentially canceling said magneticfield on the opposite side of said tool.
 12. The method of claim 11further comprising: disposing ferromagnetic material within each of saidholes of said hole antenna elements; terminating each end of said holeswith non-conducting inserts; forming a slit extending radially outwardfrom said hole to an outer surface of said antenna housing; andperpendicularly traversing each said slit with said antenna wire. 13.The method of claim 12 further comprising filling said slit withnon-conducting wear resistant material.